Sometimes more-precise measurements are all we need to make new discoveries in old structures! In a new study, data from the Gaia mission has revealed a surprise hidden among main-sequence stars.Old Diagram with a New FeatureAn example HR diagram containing 22,000 stars from the Hipparcos catalog and 1,000 from the Gliese catalog of nearby stars. Main-sequence stars are visible as the diagonal band across the diagram, but data is sparse for M dwarfs in this plot (lower right corner). [Richard Powell]If you’ve ever taken an introductory astronomy class, you’ve probably encountered the Hertzsprung-Russell (HR) diagram: a diagram on which stellar luminosities are plotted against their colors, which serve as a proxy for their effective temperatures. The resulting positions of stars on the HR diagram reveal distinct stellar evolutionary stages — and perhaps the most striking population is the swath of main-sequence stars that cuts diagonally across the diagram.Though we’ve constructed HR diagrams for nearby stars for more than a century, they continue to change as our data for these stars improve. In particular, today’s era of precision astrometry has significantly improved the distance measurements for the stars that surround us, allowing them to be placed more accurately ...